Uneven UV-unwrapping
You should choose another unwraping method(just after unwrap operation at the bottom left corner of the 3d View). 'Conformal' seems to work fine.



Also, you should not use 2.80 version. It is not released yet. Bugs are still very much expected in the beta version so it will most likely cause various problems for you especially while you are learning and might not understand what should be happening and what might be happening because of bugs. At least keep this in mind because some things are going to work in ways you are not even supposed to expect in this version because it is still experimental.